Vent systems are employed on aircraft to perform fuel system pressure equalisation and allow the safe management of air and fuel quantities and pressure within fuel tanks. Air is drawn into the tank when fuel is consumed, during descent and jettison. Air passes out of the tank during refuel and ascent.
FIG. 1 illustrates a schematic vertical section view of a prior art aircraft fuel system having a ventilation system. The fuel system includes a three-tank configuration including left and right lateral wing tanks 1, 2, and a centre wing tank 3. The ventilation system includes left and right vent-surge tanks 4, 5; a vent line 6 connecting the left lateral wing tank 2 to the left vent-surge tank 4; a vent line 7 connecting the centre wing tank 3 to the left vent-surge tank 4; and a vent line 8 connecting the right lateral wing tank 1 to the right vent-surge tank 5. The vent-surge tanks 4, 5 each include a NACA duct 9, 10, which opens to ambient from the lower aerodynamic surface adjacent the wing tips 11, 12.
The vent lines 6, 7, 8 are open at each end and allow the passage of air and/or fuel to the vent-surge tanks 4, 5. The vent lines 6, 7, 8 connect the fuel tank ullage (the space above the liquid fuel in the tanks 1, 2, 3) to ambient via the vent-surge tanks 4, 5 and their respective NACA ducts 9, 10. In the event of overfilling of the fuel tanks 1, 2, 3, or manoeuvring with full tanks, excess fuel can pass along the vent lines 6, 7, 8 into the vent-surge tanks 4, 5. The vent-surge tanks 4, 5 retain fuel surge and prevent ejection overboard.
The aircraft fuel system shown in FIG. 1 is for an aircraft with a dihedral wing configuration. Aircraft with a dihedral wing configuration commonly have the vent lines 6, 7, 8 descending from the wing tips 11, 12 toward the fuselage 13. The vent lines 6, 7, 8 connect between the upper part of the fuel tanks 1, 2, 3 and the upper part of the vent-surge tanks 4, 5.
The vent-surge tank 4, 5 capacity is proportioned according to the volume of the vent lines 6, 7, 8, to accommodate fuel which spills into it during manoeuvring or refuel. Each NACA duct 9, 10 has a vent inlet of NACA form to give ram pressure recovery to the tanks 1, 2, 3 and fuel system. The NACA duct assembly also accommodates a flame arrestor to prevent external ignition sources from entering the fuel system.
Additionally, the fuel system typically includes a fuel scavenge system, a plurality of drain valves, check valves, float vent valves, overpressure burst discs and pipe geometry.
Water is an unavoidable contaminant in fuel. It can affect fuel system component reliability and lead to operational delays and increased maintenance activities. In addition, the propensity for microbiological contamination is directly proportional to the presence of water and the temperature within fuel tanks. Sources of water in aircraft fuel tanks is from fuel loaded into the aircraft fuel tanks during refuel (dissolved water) and from air entering the aircraft fuel tanks via its ventilation system. It is estimated that up to 50% of water in the fuel of aircraft fuel tanks is currently entering via the ventilation system, depending on atmospheric conditions.
During cruise, the fuel level decreases steadily as the engines consume the fuel. A decrease in the fuel level causes an increase in the ullage volume, and excess air is drawn in from ambient via the ventilation system to equalise pressures. At cruise, the ambient air is relatively cold and dry.
During descent, due to increasing ambient pressure as the aircraft descends, air contracts in the ullage. This results in a net inflow of ambient air through the ventilation system. The ingress of ambient air during descent brings relatively warm, humid air into the fuel system, and hence a significant volume of water enters the fuel tanks via the ventilation system.
Referring once again to FIG. 1, the vent lines 6, 7, 8 include a plurality of one-way duck-bill valves (not shown) along their length. Water condenses out of the air in the vent lines and the one-way valves allow this water to freely drip into the fuel tanks. The fuel system also includes a scavenging jet pump (not shown) in each of the vent-surge tanks 4, 5, which extracts fuel and/or water from the vent-surge tanks 4,5 and returns this to the fuel tanks 1, 2, 3.
As can be seen, conventional fuel systems are designed to direct a significant amount of water into the fuel tanks. Water in suspension in the fuel will ultimately be fed to the engines to be “burnt off” with the fuel, but some of the water in the fuel tanks will condense out and pool at the low points of the tank(s). Water drain valves in the tanks require periodic maintenance to drain off the water, which is costly and time consuming. In addition, there is a risk of the water turning to ice, which can affect fuel system component reliability.
The Airbus A380 aircraft has a vent system configured differently to that described above with reference to FIG. 1, with the vent and surge tank being segregated from one another. The reason for the difference is not related to water management but to wing bending relief. The surge tank is positioned further inboard than the vent tank which remains at the wing tip. The Airbus A380 is currently certified and in-service with this un-orthodox vent system.
US2005/0241700A1 presents a proposed “improved fuel storage and venting system” with a venting chamber located central to the aircraft and surge tanks located at each wing tip. Fuel surge/overflow is fed via the central vent tank to each respective wing surge tank. This document describes that the vent system may be applied to a dihedral wing—however the application has been designed for application to an anhedral winged aircraft. The venting system is also inerted using external air as source.
There is a need in the art for a system for improving the management of water within aircraft fuel systems, more specifically aircraft fuel system tanks and venting systems.